This invention relates in general to kettles for heating asphalt, and relates in particular to improvements in temperature control and gas fuel burner pilot flame apparatus used with such kettles.
Asphalt heating kettles are known, and are typically used in various applications where a limited supply of hot liquid asphalt is required. The construction and repair of built-up roofs is one such application. The asphalt kettles in simplest form include a tank or "kettle" for receiving and holding a quantity of asphaltic material, and a suitable heater such as a propane gas burner for heating the kettle. The kettles are usually equipped with wheels, and are easily towed to the job site.
Asphalt is a solid material at normal room temperature, and must be heated in order to become sufficiently liquified for practical application. Portable asphalt kettles are typically provided with a gas fuel burner fired from a self-contained supply of gas such as propane or the like. The gas flame from the burner passes through one or more flues extending through the asphalt-receiving kettle. The heat of these flues liquifies the solid asphalt introduced into the kettle, and then maintains the liquified asphalt at an elevated temperature sufficient for the intended application.
Certain problems associated with burner control have arisen in the practical use of prior-art asphalt kettles, because the asphalt must be maintained within a range of temperatures generally not exceeding approximately 450.degree. F. If the asphalt temperature falls below the desired range, the asphalt thickens and lacks sufficient fluidity for easy application. If the maximum temperature within the kettle is improperly controlled, the kettle temperature will exceed the flash-point temperature of asphalt, causing the asphalt to ignite within the kettle. The resulting asphalt fire can damage the kettle unless promptly extinguished, and may injure inexperienced persons attempting to put out the fire.
The problem of maintaining proper temperature control in asphalt kettles is made more difficult by the fact that kettle user-operators are typically inexperienced, and often fail to appreciate the hazard of overheating an asphalt kettle. The kettle gas burner is typically operated at a particular setting when the kettle is initially filled, and allowed to operate at the same setting as asphalt is withdrawn from the kettle. The burner continues to supply the same amount of heat to the kettle while the heat-absorbing asphalt is removed, frequently allowing the kettle temperature to reach the flash point as the kettle becomes empty.
Prior art attempts for thermostatic control of asphalt kettle burners have been less than satisfactory. As noted above, the amount of heat required by the kettle varies with the amount of asphalt in the kettle. Furthermore, merely placing a temperature sensor such as a thermostat bulb in the kettle will be relatively ineffective during cold start-up, where the kettle contains solid asphalt at ambient temperature. Asphalt in contact with the heated flue will rapidly melt and may become overheated, even though the layer mass of asphalt in the kettle remains unliquified.
Past attempts to provide thermostatic control for gas fuel burners in asphalt kettles has produced the related problem of pilot flame blowout. The conventional gas burner includes a main burner typically providing a relatively high velocity flame aimed in a generally horizontal direction to enter the flue extending through the asphalt kettle, and also includes a pilot burner directing a pilot flame into a region directly in front of the main burner. The pilot flame is ignited at the start of kettle operation, and this flame burns continuously as the main burner turns on and off in response to thermostatic control.
When the gas supply to the main burner is turned on in response to the thermostat, the volume of gas flowing through the high-velocity main burner is ignited by the pilot flame. This ignition frequently causes a sudden blast or shock wave caused by the sudden ignition of gas from the burner. The effect of this blast is intensified because the burner assembly is housed in an enclosure of relatively small volume, and the blast frequently blows out the pilot flame. Although safety devices are typically provided to shut off the gas supply in the absence of a sensed pilot flame, the asphalt kettle and its contents are no longer heated. The absence of burner operation will typically be discovered only some time later, after the kettle has cooled and the asphalt therein has commenced to hardened. The burner must now be relit, while the work crew remains idle waiting for the asphalt to be reheated to a usable temperature.